Appropriate Applications for Lightweight Concrete

Fernando Martinez Lewels has a M.S.C.E degree from the University of Texas at El Paso. He is now working with the Agartif company in Chihuahua, Mexico (about 170 miles from El Paso Texas). This Company has developed a type of lightweight aggregate that provides material for all types of construction needs, at reasonable cost and with good thermal insulation values. They manufacture the equipment required to do this according to the needs of their customers; the feed stock are common construction materials that should be available in most locations. Their philosophy in developing this type of aggregate is to be able to use this everywhere, without depending on a lightweight aggregate quarry, so you can have access to this material in any part of the world. In Mexico we have a saying that "we build our homes so we have to go outside in the summer to be fresh, and in the winter we go outside to catch the rays of sun to be warm". Lightweight concrete can help this situation by making available materials for more comfortable homes.

Questions and Answers

Q: I have an existing external deck with the floor boards consisting of what I believe is referred to as "flex-boards". My question is whether or not lightweight concrete can be applied on top of the flooring (providing an edge were to be installed to keep from running off) to serve as a water-proof ceiling & flooring? My intent would then be to lay ceramic tile on top of the cement. I did consider regular concrete, but was informed that it would not be a good idea to pour cement over wood. Also, my builder assures me the foundation structure can support the weight.

A: (Scott MacHardy) I wouldn't think it a good idea to use any kind of concrete over wood deck materially like that, just because of the rot factor on down the road.

Q: I want to make my own lightweight concrete as a slab for radiant floor heating because of the much lighter weight than regular concrete.
Using Vermiculite, what would you suggest the mixture of of mortar / sand / Vermiculite should be? The floor will be finished with ceramic
tiles, and the water temperature in the heating pipes is about 140F maximum.

A (Kelly): The use of any lightweight concrete for a radiant floor system would not be advisable, because the heat from the pipes needs to be able to permeate the entire mass and be radiated into the tiles and the room. Lightweight concrete tends to be insulating material which resists the transfer of heat, so such a system would not be very efficient. You would be much better off to use ordinary concrete for this purpose.

Q: I live in Florida with high humidity and the dangers of high winds from tornados and hurricanes. I am interested in building a home using a poured concrete method. Can the lightweight concrete stand up to high winds ?

A: (Kelly) I would think that many lightweight concrete formulas would stand up fine to the weather, especially if you are imagining domed shapes, which are particularly sturdy in the wind.

Q: I was wondering if the lightweight concrete can ever be made light enough to be used as furniture ? How heavy is it compared to solid hardwood furniture?

A: (Kelly) I haven't actually heard of anybody doing this, but that doesn't mean it couldn't be done. It would take some experimentation. I have heard of people making outdoor tables out of concrete that stand up to the weather very well.

Q: I live in New Hampshire and am looking for a product to use for building a full walkout basement. Is pumice concrete mix a good alternative to a standard poured foundation? Also, need details like cost, r value, etc. I look forward to hearing from you. Going to build in two months so need to make a decision soon. thanks!

A: (Kelly) In general I would say that pumicecrete would be a good choice for a poured basement/foundation because it is relatively insulating (I don't have figures for R-value). The building officials in your area might require regular cement, however, because it has known compression qualities that relate to code requirements.

Q: We are a custom cabinet shop experimenting with concrete counters and are searching for a better/lighter product than standard sand mix. Any advise on mixtures? Will a lightweight concrete be too porous? What is the thinnest application, would 1.5" be workable?

A: (Kelly) The trouble with many lightweight aggregates for concrete is that they tend to be rather porous and/or soft, so that durability of the surface could be an issue. I would suggest some experimentation with various materials, thickness, and possible finish coatings that might make for a harder surface. It does seem possible to me that some combination of things would work well.

Q: I am considering two different designs for a geodesic dome (about 40' diameter) that involve lightweight concrete. The first is to use a rebar/mesh form, which seems pretty straight-forward and do-able. The second (and more interesting) possibility is to make forms for the triangular panels, and mortar the poured panels together to form the dome. Would you have any feedback on the feasibility or drawbacks of these methods?

A: (Kelly) As you say, the first method is fairly straight-forward, and has been done many times in many different ways. Monolithic domes are basically done this way. I have a neighbor who single-handedly made his home this way, mixing all the concrete by hand (and he is in his 60's). The advantage of the approach is that you end up with a more monolithic structure that is less likely to have leakage problems, and the process of building is probably easier.

Your other suggestion I see fraught with difficulties: Geodesic structures are not simple (there are several different triangular shapes to any geodesic), and require very precise measurements to fit together . Even light-weight concrete is heavy, so you need to figure out a way to erect the thing once the triangles are cast. The biggest problem with most geodesic domes is leakage between the segments, and your cement mortared joints would likely be no exception. All-in-all, I would think that this method would take you about three times as long to accomplish, with less satisfactory results.

Q: I need a lightweight version of a limestone fireplace surround mantle. They are now producing cast stone ones from concrete. Would any lightweight product work? It would have zero clearance to the firebox? Would it be okay with the heat?

A: (Bruce Schundler) Not all lightweight concrete products are made from stone aggregates---and not all are immune to high temperatures. Nevertheless, if the aggregates are lightweight mineral, inert and fire-proof aggregates like perlite or vermiculite or other aggregates like pumice or expanded shale, then the concrete probably would be OK and could withstand a lot of heat. The key would be whether there is air entrainment, and whether the concrete is designed for any kind of refractory application. And if the mantle doesn't get particularly hot, then almost concrete made with mineral aggregates would be OK(e.g. again, not polystyrene or other organic aggregates).

Q: I'd like to make stair treads inlaid with natural stone. Local contractors seem lost. Specifically I'd like to make 2.5" x 11" x 60" treads, reinforced with 3 1/2" rebar. I'd like to press 1/2" thick ledgestone into the wet concrete. These 11 treads to be supported by 1/8" x 2" x 2" galvanized angle iron brackets bolted to three 4" x 10" stringers, no risers. This is in southern California, More details than you need. What lightweight concrete formula is strong enough to support normal residential traffic on such a stair tread? Can you suggest a local source for materials?

A: (Bruce Schundler)Usually lightweight concretes would not be used for this. They don't have the same structural or bonding strengths as regular concretes. But if weight really is an issue, a fairly strong 1 to four mix (one part Portland cement to four parts perlite or vermiculite) with fiber reinforcements might work.

Q: Hi, I am in a watershed area in North Carolina. I purchased a home a few years ago and would like to expand the driveway. My problem is that I have a limit to the amount of impervious area I may have on my property. I have researched it and am not able to work around this. I was at a standstill when someone told me to check into porous concrete. Can I use this to extend my driveway? If so, should it meet the porous requirements? And if so, will it look greatly different from the traditional concrete, already in place?

A: (Kelly) I'm not sure that lightweight concrete would meet the standards for allowing water to drain through it...I suspect that it would not. My recommendation would be to use pavers that are designed for this purpose.They come in a variety of patterns and are just assembled like patio pavers, but with more open space to let the water drain. They would not look just like concrete, but they are quite pretty.

Q: Your advice to not use any lightweight concrete to use as a thin-slab for a radiant heating system contradicts the information on many websites, including, of course, manufacturers of supplies (pex tubing and "liquid" lightweight concrete or gypsum). I am at a loss when I read experts contradict each other so flat out, but you have no vested interest, so...thanks for the website.

A: (Kelly) You are right that I have no vested interest in this question, nor would I necessary consider myself an expert. I am really just relating what seems like common sense to me. Radiant floors need to radiate their heat, and anything that interferes with that process (such as insulating the heat tubes from the room) would lessen their effectiveness. Also, the more thermal mass involved, the better the system will retain heat over time and moderate temperatures in general. I can certainly see using lightweight concrete beneath the heat tubes to insulate them from below, and then using a denser material above that to actually embed the tubes. Or, if weight is an issue, then lightweight concrete might be a choice of compromise. I know that radiant tubes can be installed with wood floors, which would be somewhat insulating, but again I would expect the performance to be less efficient and comfortable than using a better thermal mass material. If you encounter any tests that have been done to prove any of this, please let me know.

Q: I live in Southern California where it is very hot during the summer. My asphalt shingle roof is beginning to crack and we had a serious leak during heavy rains in December. I have been looking at many different types of material to replace the shingles. This would entail removing all existing shingles and taking them to the landfill. I have been looking at different versions of concrete and am wondering if putting a 1 inch layer of lightweight concrete on the roof using an artistic design is possible? There are elastomeric coatings with very high UV radiant capabilities that can be painted on to waterproof and insulate the coating. What is your opinion?

A: (Bruce Schundler) Perlite concrete has been used very successfully---both on flat roofs, pitched roofs, and in roofing tiles. And there is one company in California that has developed a roof deck that incorporates a thin perlite layer on top of the regular water proofing of a flat deck, and some of these decks have lasted 40-50 years.What can be done will depend a little on the slope of the roof, and on the structural integrity of the roof. There are three perlite manufacturers in California, and the one in North Hollywood (Redco II--- www.perlite.net ) has been involved with perlite concretes for years. They might have some suggestions and might be able to help.

Q: I want to make my own balusters for my deck. I live in the northeast corner of Wyoming, so we have a freeze thaw factor. I would like to know if lightweight concrete can be poured into molds and if it would hold up in my climate? If so, please advise a formula.

A: (Bruce Schundler) Probably lightweight concrete should not be used. Balusters normally are used to prevent children, dogs, and very short people from falling off decks and staircases. As such, their function is more like a fence, and they need to be able to withstand horizontal and lateral forces, and a fair amount of abuse. Regular dense concrete would not be good for this, and if used, would need reinforcing rods or rebar to make the baluster as strong and safe as it should be---and in this application, lightweight concrete would be even worse in terms of strength, durability, etc. Hope this helps (e.g. hope you don't try to use lightweight concrete), and good luck!

Q: I'm building affordable housing from sip type panels in Latin America, tropical. I'm looking for a concrete base material recipe that is lightweight for a floor of 12' x 20' room. I plan to pour the floor after the house is erected and can use the inner walls as part of my form. The floor will have very light loads. I would like to keep thin and low cost for the market. In some cases, the house will be on pylons off the ground and will be suspended so I would use reusable forms under the floor while pouring. I have available a sisal type natural fiber I was thinking of using as a filler. I would appreciate any thoughts or suggestions you may have.

A: (Bruce) Perlite and vermiculite concretes are used for floor fills and under some floors, but these floors usually are covered with something after curing. The problem is that perlite and vermiculite concretes tend to be fairly "soft" and cannot withstand scuffs, hard scratches, etc.As a result, they are covered with carpeting, tiles, wood flooring, etc. The end result is a very insulating floor, that also resists noise transmission through the floor. They also aren't good for a suspended floor that would require some structural function. Essentially perlite and vermiculite concrete could not be used and shouldn't be used for a floor that is above grade unless it is poured on a surface that can withhold all the loadings normally required of floor designs. Having said all that, they could be used if placed on or over the ground, if the aggregates were readily available from local sources, and if an extremely hard surface is not required.

Q: I recently produced a concrete replica of a human posterior a a joke for a friend. We left the center hollow and provided a hole to allow the posterior to be use as a cigarette butt collector. Numerous people have displayed interest in having one of these "Butt" cans for their patios. Originally I used regular 5 bag with 3/4 minus aggregate. As you can imagine this was really heavy. I was wondering if you could recommend a reasonable priced LWC that would exhibit really good heat resistance wile lowering the unit weight to around 35 to 70 pounds/ft3. The LWC would also have to be moldable and castable.

A: (Bruce) Perlite could be used and the possible mix designs are all on our website (schundler.com) in the "construction" section. Interesting use.....

Q: Can your product be used to pour a new driveway over a old one ? Half the old driveway is cracking and falling apart.

A: I can see no advantage to using pumicecrete over an old driveway, since it might not hold up to the wear as well as standard concrete.

Q: We have an aircraft hangar that has a 6 foot wide and 58 foot long deck. It’s a metal structure covered with ¾ inch plywood. The plywood is covered with a heavy neoprene roofing material. We used indoor/outdoor carpet (glued down) on top of the neoprene roofing; unfortunately it deteriorated after a few years and it was a lot of work getting it off the deck. We are now looking for other options. We want to protect the deck from leaks because it is over our shop area. The deck covers about 3 feet of shop area and extends out another 3 feet; the deck is 12 feet off the ground. During our annual air show the deck receives a lot of traffic. We have discussed using carpet, but would prefer a longer lasting solution. Could light weight concrete tiles be used on the deck? How heavy our the tiles and what shape/size do they come in?

A: (Schundler) Light-weight concrete is used regularly in building as well as restoring flat roof decks. As part of a complete system, perlite concrete can be feathered out to re-angle or fill low spots, but must be protected from the elements, i.e. water, ice and ultraviolet rays. If heavy traffic occurs you can refer to mix designs to 'beef-up' the psi. We only manufacturer to aggregate. As for light-weight concrete tiles, check with the manufacturer for their psi.

Q: I am interested in light weight concrete which can be use as paving block because I think I can use it in my project.

A: (Kelly) Light-weight concrete can obviously be formed into blocks for use in paving projects, but this would not be my first choice of materials for this purpose, because such blocks would not be as durable over time as denser masonry products.

Q: I am building a 6 ft x 6 ft shower with a mortar (concrete) base. The bathroom is on the 2nd floor. What do I need to use (and in what proportion) to lighten the weight of the concrete? The lighter the better but I also need strength.

A: (Kelly)You can replace the stone aggregate in concrete with crushed volcanic rock, or perlite to make it lighter for your shower pan. I would suggest using  wire mesh to reinforce it as you would for ordinary concrete. You might experiment with different mixes to see what works best. Also, you may want to use a tough paint to keep it from absorbing moisture.

Q: I make large expanded polystyrene spheres with Hexagons and Pentagons, up to 6 ft. in diameter. I would like to make a large dome about 15 ft. in diameter at the base using the same technique as the spheres with the Pentagons & Hexagons. I envisage coating the outside of this shape with some cement mix; somebody suggested adding Vermiculite to the sand & cement mix? I guess I would also encapsulate the cement coating in a steel or mesh frame around the polystyrene shape. Can you suggest a suitable mix that is light but strong please.

A: (Kelly) You might look at the formula suggestions at Formulas. In general, I am sure you will have to do some experimenting to see what works well for your application. Besides using wire mesh reinforcement, you might experiment with the commercial fiber reinforcing material that it used in the concrete industry these days....it would probably be a lot easier to work with.

Q: We want to produce large outdoor sculptures that will be displayed in a public place. People will be sitting, standing, or walking on them, so I am wondering if lightweight concrete is the material to build the sculptures with.

A (Kelly): Light-weight concrete can be a very sculptural medium, and for that reason it might work well for you. On the other hand, it is not as scuff resistant and durable as ordinary concrete, so I would recommend that you stay with this.

Q: My application is for fireproofing gable-end fascia boards on a high-roofed chalet I am building in the midst of the foothill Sierra Nevada mountains in California. I have a unique shape to my fascia boards, which are now formed with 1/2" thick OSB clad to 2x10 rough framing fascia boards. My overhang is 2 feet. I will be boxing in under the overhang and eaves and stuccoing the house. On the fascia boards themselves, I want to clad about 3/4 inch to 1 1/2 inch of lightweight concrete that will be exposed directly to sunlight and weather. I can (will) use chicken wire to cling the concrete to the fascia boards. I will be applying the job myself with a small mixer. What lightweight concrete do you recommend for this job, and how to I find the mix ratio formula?s This house is a completely do-it-yourself, earth-friendly home.

A (Kelly): I actually think you would be better off using a standard stucco mix for the application that you describe. It will likely be easier to mix and apply, and more durable than something of less weight.

Q: I have been interested in lightweight concrete as a building material for quite some time. I am wondering about a more "conventional" use, i.e.: sound deadening in sub floor construction. I need to patch an existing floor where we have had to make a few openings. Also, I would like to apply a layer in my own condo before adding a wood floor. I believe it is possible to hand mix small quantities of lightweight concrete. This would enable a reasonable patch job for the existing floor. My main question is what might be the thinnest practical dimension of the membrane for a sub floor using any "newly available" materials? I need to keep it thin, thin, thin. I have seen a few lightweight sub floors. These, including my patch job seem to be around 1,1/2" but I can't go this thick for my own floor.

A (Kelly): It seems that lightweight concrete is fairly commonly used as a subfloor, and I would imagine that 1 1/2" is about as thin as it would normally be. You might experiment with adding fiberglass fibers that are conventionally used to reinforce concrete to see if that might make a thinner application practical; try this in a test sample.

Q: We live in Washington State and were wondering if we could use concrete (preferable lightweight) as a countertop for our outdoor BBQ center? If so how thick would it have to be and could you lead us to a source for buying the supplies we need and maybe a web link or book to learn how to apply it and stain/texture it? My husband is a builder and is experienced with regular concrete but would need detailed instructions for this new light weight concrete.

A (Kelly): I suggest that you stick with a cement-rich ordinary concrete for this application; lightweight concrete will not be as strong and would tend to be more porous than ideal.

Q: What material would you recommend for making a decorative cover for a range hood. It doesn't need to hold weight, but it must be able to withstand cooking heat from a high heat stove and oven. I can make an armature out of plywood or wire. It is over an island range and will be held up from above, so weight is an issue. I am making this for my own home, but would eventually like to do this as a business.

A: I suppose that it would be possible to fashion a range hood from lightweight cement, if you were very careful about handling it. Masonry materials of this sort are inherently fragile and can crack easily. If you attempt this, I suggest that you make a wire mesh armature and try using perlite as the aggregate. This will take some experimentation. If it were me, I would fabricate the hood out of thin metal, which is commonly done and is not fought with the problems of masonry.

Q: I'm concerned that the metal and concrete will expand and contract at different rates when they are exposed to heat. Does that happen?

A: I doubt that this would be a problem, since steel-reinforced concrete is used in many applications where they get pretty hot, like out in the blistering sun, without breaking apart.

Q: Can lightweight concrete be used over radiant pipes in a home? We have wood floors with new construction.

A: (Kelly) I suppose that lightweight concrete could be used over radiant pipes, but I wouldn't recommend it because this concrete is more insulating than standard concrete, and this would inhibit the movement of the heat.

Q: I'm planning on building a concrete tilt-up home, with the goal of having a thermal mass solar home. I was interested in using a sandwich construction with normal concrete on the outside and EPS in the center, but this design seems to have some possible drawbacks including shrinkage of the insulation and termite tunneling (although they don't eat the EPS but can nest in it). I'm interested in the concept of pouring a 3.5" layer of standard concrete to form the inner wall, then pouring a lightweight concrete about 6" or more for the insulation value and exterior surface. I've heard of estimated R values of 1.5 to 2.5 per inch for lightweight concrete mixes, which could give me as much as R 15 for the 6" lightweight concrete layer. I want super insulation and R 15 sounds quite good, but will it really perform like a similar wood frame wall with that level of insulation? Any other gotchas?

A: According to the Schundler website, 3" of vermiculite concrete is equivalent to 1 1/2" of rigid board insulation, which is to say about R-7 1/2. If you double this to 6", you would end up with about R-15, which is pretty much what you quote. A standard 6" stud wall with fiberglass insulation provides about R-19, although the tilt panels would likely perform much better that this, because they are much tighter, without the thermal bridges of wood, and they have all of the thermal mass on the inside. So I think your idea would probably work out quite well.

Q: I'd like to use Lightweight Concrete to cover the floor of several patios I have which have leak issues. In my scenario, I have ripped off all the rotten plywood, replaced the joists, etc and now am ready to cover these with metal decking as opposed to plywood. This decking would then be covered with Light Weight Concrete (1.5"-2"), and then finally tile. Would this be advised or should I just stick with floor board, and tile. Also, if you do advise the Light weight concrete, should I use glass fibers, wire mesh, etc. to ensure it wouldn't crack too badly?

A: (Kelly) From my own limited understanding, I would say that what you propose is sound, as long as the joists and decking are quite firm and will not flex much with the weight. I have been quite impressed with glass fibers as reinforcement, especially since this will then be covered by tiles and any fibers sticking up will be covered.

Q: I have a flat roof/ceiling consisting of plaster interior, 2 x 10 rafters, negligible insulation, 3/8 plywood with 2" foam, covered by elastomeric. I want to create some slope and increase the insulation. What are your thoughts about applying 2" to 8" of perlite/lime/cement covered by elastomeric coating and what should I use for reinforcement and how should I bond it to the existing roof?

A: (Kelly) I know that such material is often used in roof applications like you propose. You might take a look at http://www.schundler.com/roofdecks.htm  for more information about this.

Q: I am looking to replace a wooden balustrade system with one of concrete. The cost of shipping is so high, not to mention the cost of precast materials, that I decided to manufacture the system on site by casting my own. My concern is the weight of conventional concrete. I am thinking of using a perlite or vermiculite based concrete mix to reduce the weight but am concerned about the structural integrity. Please advise your feelings on which product would be better suited to this application, if either. If neither, what would you recommend.

A: (Kelly) I must admit that the idea of casting a balustrade system with either lightweight or conventional concrete seems like a lot of work for something that may not serve you well. Even with steel reinforcement these materials are brittle and likely to crack over time. I think that either wood or steel would be a much better choice.

Q: I have a ferro-cement dome-roof building that I want to change the shape of slightly on one side. I want to add as little weight as possible in the process. Could light weight concrete work well here? That is, does it play nice and adhere well to standard cement without spalling?

A: (Paul Sarnstrom) Yes light weight concrete could work, depending on the size of the area to be modified but there are different types of light weight concrete with different weights and I have no idea which one specifically you are referring to. FC [ferrocement] might be as light or lighter than the amount of light-weight concrete required, again depending on the amount of area and what mix you use. You ask if light weight concrete would adhere to cement without spalling. I assume you are asking if light weight concrete will adhere to mortar, which is the matrix of FC. It could adhere very well or it could adhere poorly depending on the materials and techniques used.

Q: I have a driveway on concrete piers supported with timbers and posts with 1 1/8 plywood as the deck.The driveway goes from 1 foot to 8 foot above the ground. Could I pour an inch and one half of lightweight over the top of the plywood to give it a finish surface?

A: (Kelly) Lightweight concrete is often used as roof decking, but in that function it is not given the rough treatment that you would expect for a driveway. I have never heard of LW concrete being used this way, so it might be experimental. I would also be concerned about the potential for rot occurring to the subsurface wood from lack of proper ventilation.

Q: We have recently bought a well-built ugly early-seventies house, 4" walls covered in stucco with embedded sharp bits of quartz and rock. We are in a west coast mild climate much like Port Townsend, Washington and I'm wondering if it's possible to resurface the walls with lightweight concrete for a plain plaster finish and improved insulation. What preparation would be needed? (lots of the pebbles are fairly loose). And would it work to use something like bark mulch as the lightweight component -- available, and fibrous, to add integrity?

A: (Kelly) It does seem feasible to me to add a light-weight plaster to your house, but it would likely take several inches to make much difference in the insulative value of the wall. You might need to attach a stucco netting to your existing wall to assure that the new plaster sticks to it. Whether bark mulch would work as an aggregate is hard to say. It certainly would not be as durable as a mineral material, such as crushed volcanic stone or perlite. You could run some tests somewhere to see how it might hold up...

Q: I am an Australian architect working for a charity organization in Nicaragua, currently designing and building on a low budget; a small (3m x 4.5m) community radio station out of sand pack recycled plastic bottles, in a confined masonry type construction system to help stabilize the building for earth quake purposes. The site has a lot of volcanic rock. The locals inform me that we can use volcanic rock as a substitute for aggregate in the concrete mixture. Building regulations are not so strict here. The volcanic rock is not pumice, but has a lot of air pockets in it. I am unsure whether it will be stable enough for structural purposes. It could be a great cost saving and environmental benefit if it could be used. Currently I am content with using the volcanic rock in the floor slab, but uncertain about other structural places. Any information or advice would be greatly appreciated.

A: (Kelly) I would think that you should be able to use this volcanic stone as an aggregate for a lightweight concrete. A variety of types of volcanic stone has been successfully used as aggregate besides pumice, even though this general class of concrete has been called "pumicecrete". I do have some information about this shown at here.

Q: I am interested in making tiles and countertops with lightweight concrete and was wondering what a good mixture would be, and what the minimum thickness would be required. The countertops would be supported throughout. Ideally I would like something lightweight (for structural reasons), sufficiently strong to stand on (tiles) and to withstand daily countertop use, and I would also like it to be smooth. What do you recommend?

A: (Kelly) There is an art to making good durable concrete counter tops and I'm pretty sure that they use highly enriched (with cement) concrete for this. While substituting for the standard gravel aggregate a lighter weight material would cut the weight somewhat, it would also render the finished product more vulnerable to breakage or damage I suspect.

Q: Do you have any advice for concrete tiles? I’m particularly curious how thin I can make them without worrying about cracking and if lightweight concrete would be appropriate to use for this application.

A: (Kelly) Floor tiles can really take a beating.. Thin slabs of concrete are never as strong as fired ceramic tiles, so I would think they would need to be nearly an inch thick before they would work well as floor tiles. And lightweight concrete would not be as durable as regular concrete.

Q: I'm gonna be making a small cabin in a place were the climate is mild or temperate. It's in the woods and yeah... it rains a lot! I know that pumice lacks frost resistance when wet and that after exposure to water or humidity for a while it looses its insulation properties since it starts absorbing the liquid. There's pumice available in the area and I'm attracted to the idea of building with it, so the question I have is: do you think pumice is a good choice for building in that kind of weather?

A: (Robert Alexander) Pumice-crete walls are porous, so a finish coating which will help the walls shed as much water as possible is necessary. Here in New Mexico, where the climate is generally dry, a thin (1/8" to 1/4" thick) "brown coat" of cement and sand is applied directly over the pumice-crete wall, followed by a cement based stucco color coat. Often these are applied by hand using a trowel, but they can also be sprayed if that equipment is available. Wiring the wall is not required as the brown coat, being cement based, sticks well to the pumice-crete.

If necessary, it may be possible to apply a water repellent over that - or possibly add it to the color coat - but I have no experience with that and the effect would likely be to stop or reduce the wall's ability to transpire moisture from within, so additional interior venting would probably be required.

Another idea might be to extend the roof overhangs and, if practical, install a gutter system to protect the walls from back splash. I can say that, in general, I consider pumice-crete to be one of the best building materials I have worked with in more than twenty years of researching and working with sustainable materials.

Q: What if lightweight concrete that uses polystyrene is used as a wall panel? Is it appropriate?

A: (Kelly) Lightweight concrete with polystyrene could be used for wall panels that do not bear weight. They may need some wire mesh reinforcement to hold together over time.

Q: I am wondering if I can use lightweight concrete to sculpt a soaking bathtub? What thickness and what type of reinforcement would be needed (three sides will be against a wall)?

A: (Kelly) A bathtub would ideally be as sturdy, waterproof and resistant to abrasions as possible...and none of these qualities are found with lightweight concrete. Even if you use ordinary concrete, I would expect it would have to be perhaps 3" thick and be heavily reinforced with wire mesh to be very durable.

Q: Have you any ideas or plans on building fences, like six feet high around one's property? Do you think that the pumicecrete would work OK and be cheaper than block cement? Wood fences are a pain to keep up and the weeds and Bermuda grass from the neighbors goes under chain link.

A: (Kelly) You could make an excellent fence with pumicecrete. I don't know if it would be cheaper that concrete blocks, but it would probably look nicer.

Q: Can lightweight concrete be applied to outdoor cinder block for a plaster type finish and stick well? Also, can lightweight concrete be mixed with concrete dye or stain before application?

A: (Kelly) Lightweight concrete is not usually used for plastering applications; it doesn't tend to be quite as durable a finish because often the aggregate is softer, or there is more air entrained in the mixture. If the mixture were adhesive enough it should stick OK, so you might add lime or even latex. I suggest experimenting to see what works well. I see no reason why such lightweight concrete couldn't be colored with the same materials used with ordinary concrete.

Q: We live in a hi rise and want to install 20x20 inch porcelain tiles which are heavy. Here's the problem: We are required to have a sound barrier or underlayment. The underlayments are too expensive and more for the labor. Our sub floor is concrete slab. To muffle sound, could we put a layer of light weight concrete (for sound barrier) followed by thin set. Or, add perlite to ordinary thin set, or "something".

A: (Kelly) I think that your idea of laying light weight concrete as an insulating underlayment is good. Roof decks are often made with light weight concrete, for similar reasons. I wouldn't add perlite to the thin set though, as you don't want to compromise the strength or make it at all lumpy.

Q: I have a question about building an outdoor fire pit with concrete. What lightweight aggregate would you recommend using? I was thinking Perlite but was hoping to get an expert's opinion.

A: (Kelly) I don't have any experience using lightweight concrete in that application. It would seem to me that you would be pretty safe using scoria as an aggregate; it has certainly been through the fire. Perlite has already been expanded by heat during its manufacture, so it might also be OK. You might query a manufacturer or supplier about this.

Q: Is it possible to mix a self leveling lightweight floor screed as a DIY project at home with limited equipment; mixer shovels etc. I have a 150mm concrete sub floor on which I am going to lay 80mm insulation. I propose fixing under floor heating pipe work to insulation and covering with approximately 50mm self leveling screed. I imagine an aerated screed or a foam concrete would do the job and have great thermal properties. What mix would I use and how does one produce the foam for foam aerated screeds or how else could I easily produce a lightweight self leveling mix.

A: (Kelly) It shouldn't be too difficult to lay a level concrete floor over your heating pipe work if you are reasonably familiar with pouring concrete floors. But I would advise using a standard concrete mix rather than a lightweight one, because you really want the thermal mass of the the concrete to hold and transmit the heat from the pipes and not insulate them.

Q: I am planning to produce a 2"x 5'x10' island countertop that will be shop poured (upside down). As this will be an extremely heavy piece, I would like to cut the weight as much as possible. The top will overhang the cabinet about 12" on all 4 sides.The first layer (3/8 - 1/2") is a proprietary blend to produce the intended color. Can I use a lightweight mix (perlite) for the back coat? If so, should I reinforce with fibers or rebar?

A: (Kelly) I would think that some perlite or other lightweight cement additives could be used. I would definitely reinforce it with fiberglass fibers or mesh, perhaps stucco netting. I suggest trying a small sample in advance to make sure it works.

Q: My understanding is that Hempcrete requires a supporting frame?

A: (Kelly) Yes, hempcrete is usually used in conjunction with a wood framed wall which gives it support and gives the wall structural bearing strength; adding the hempcrete increases the overall strength of that wall. I don't know the psi rating for hempcrete, but this somewhat a moot point since it normally isn't used for bearing weight.

Q: Does hempcrete really petrify and take the strength of stone?

A: (Kelly) Lime does get harder over time but I doubt that hempcrete ever gets as hard as stone.

Q: I also have read that the plant material used HAS TO BE TREATED to stop/slow-down rotting? Is that true for Hemp?

A: (Kelly) Hempcrete is very stable under a wide range of climatic conditions and does not rot when damp.

Comment (Chris Steen): A serious aircrete mixing setup is pressurized and continuous. No paddle bits in barrels. The green dragon is diy. But a commercial style system is easily fabricated. And don't expect to save any cement usage with aircrete. There are advantageous uses for this material (like filling awkwardly shaped voids between ferrocement barrel vaults in order to properly exterior insulate). But if aircrete is not saving any cement, I suggest an insulated masonry or earthen system if appropriate.

Q:   I would like to use LW Concrete as the foundation for my house, which will be build over water, on concrete pilings, over steel I-beams, and poured into corrugated deck pans with re-enforced rebar. The intend use is for weight savings as the residential house will be basically built completely over water. There is a driveway and garage. My concern is, can one use LW Concrete (110 lb. per cbf) throughout and drive over it as well? If cars cannot drive over LW concrete, can I mix it with a top layer of standard concrete (say 2 inches), and bottom layer LW? Assuming mixing the two into a sandwich will save some weight. Also, it's assuming that LW can be mixed to have same psi strength as standard concrete.

A: (Nabil Taha) Properly proportioned and properly mixed LW concrete could be used over the corrugated metal deck for residence and drive way etc. However, If your drive way is on dirt, use regular concrete. Just make sure that you have professionals who are familiar with LW concrete  to guide you through the process.

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